At least one observation
You need to choose at least one option for your mind: Observing an inner image, listening to your inner voice or sensing an inner feeling. It can be any composition of the three, with a maximum of all three. You can create your own favorite style, for example an image of a crystal accompanied by the mantra 'Samma Arahang' and feeling some energy at your center. Some may prefer just only the feeling in their abdomen. Some may love to repeat their own personalized, and so on.
Any way that makes you feel comfortable, relaxed and at the same time able to continuously focus on the middle area of your body is the right way for you.